Method for thermochemical production of hydrogen from water

ABSTRACT

A method for the thermochemical production of hydrogen from water is disclosed in which barium iodide, carbon dioxide, ammonia and water are allowed to react with one another and give rise to barium carbonate and ammonium iodide, the ammonium iodide thus produced is thermally decomposed to produce hydrogen, iodine and ammonia, and the hydrogen thus produced is recovered as the product. The by-produced barium carbonate is allowed to react with the iodine remaining after the separation of hydrogen thereby to produce barium iodide, carbon dioxide and oxygen, and the barium iodide and carbon dioxide are recycled to the reaction system. The ammonia which remains after the separation of hydrogen is also recycled to the reaction system. By causing the by-products occurring in the various reactions to be recycled to the relevant reaction systems, hydrogen is efficiently produced from water at a reaction temperature of not more than 800° C.

BACKGROUND OF THE INVENTION

The present invention relates to a method for the thermochemicalproduction of hydrogen from water.

Establishment of a method for producing hydrogen by thermallydecomposing water at temperatures below 1000° C, preferably below 800°C, and using as the heat source solar heat or heat from an atomic pileand as the medium such compounds as are available abundantly from thestandpoint of natural resources is an important task to be fulfilled assoon as possible in view of the unavoidable exhaustion of fossil fueland the consequent necessity for securing the source of secondary energyto take its place.

In the circumstances described above, there have heretofore beenproposed 60-odd reaction cycles for the production of hydrogen bythermochemical decomposition of water.

Argonne National Laboratory, for example, has proposed a reaction cycleconsisting of the following three steps:

    LiNO.sub.2 + I.sub.2 + H.sub.2 O → LiNO.sub.3 + 2HI 1.

    2HI → H.sub.2 + I.sub.2                             2.

    LiNO.sub.3 → LiNO.sub.2 + 1/2O.sub.2                3.

This reaction cycle has not yet been developed to the extent ofcommercial application, because the reaction of Formula (1) proceedsslowly and the reaction of Formula (3) may possibly produce LiO₂, ahighly corrosive compound, under certain reaction conditions.

Euratom has proposed a reaction cycle which consists of the followingthree steps:

    6FeCl.sub.2 + 8H.sub.2 O → 2Fe.sub.3 O.sub.4 + 12HCl + 2H.sub.2 1.

    2Fe.sub.3 O.sub.4 + 3Cl.sub.2 + 12HCl → 6FeCl.sub.3 + 6H.sub.2 O + O.sub.2                                                   2.

    6FeCl.sub.3 → 6FeCl.sub.2 + Cl.sub.2                3.

An experiment has shown that the reaction of Formula (2) in thisreaction cycle does not result in any discernible generation of oxygen,indicating it difficult to have the reaction proceed through the cyclerepresented by these formulas. The reaction of Formula (3) isproblematic in terms of process and heat balance because the heat ofsublimation of FeCl₃ is great and, worse still, the ratio ofdecomposition is small.

As described above, the reaction cycles proposed to date invariablyentail problems of some sort or other and, in their existing status, aredifficult of commercial application.

An object of the present invention is to provide a method for producinghydrogen by allowing water to be thermally decomposed efficiently attemperatures of not more than 800° C through a combination ofcommercially quite feasible elementary reactions by using, as themedium, elements or compounds which are available abundantly in natureand which do not exhibit much toxicity and corrosiveness.

SUMMARY OF THE INVENTION

The present invention accomplishes the object described above byproviding a method for the production of hydrogen by the thermochemicaldecomposition of water, which method comprises (a) a step of causingbarium iodide, carbon dioxide, ammonia and water to react with oneanother to produce barium carbonate and ammonium iodide, (b) a step ofallowing iodine to react upon the barium carbonate produced in (a) stepthereby producing barium iodide, carbon dioxide and oxygen and (c) astep of thermally decomposing the ammonium iodide produced in (a) stepthereby producing hydrogen, iodine and ammonia, with said reaction cyclecontinued while causing the barium iodide and carbon dioxide produced in(b) step to be recycled to (a) step, the ammonia produced in (c) step tobe recycled to (a) step and the iodine produced in (c) step to berecycled to (b) step.

Further, the ammonium iodide obtained in (a) step is caused to reactupon tri-iron tetraoxide to produce ferrous iodide, ammonia, water andiodine. The ferrous iodide thus produced is allowed to react with waterto give rise to tri-iron tetraoxide, hydrogen and hydrogen iodide. Thehydrogen iodide is decomposed into hydrogen and iodine. The by-producedtri-iron tetraoxide, ammonia, water and iodine which remains afterrecovery of hydrogen are recycled to the relevant reaction systems.

In addition, the ammonium iodide obtained in (a) step is allowed toreact with magnesium oxide to produce magnesium iodide, ammonia andwater, and the magnesium iodide thus produced is allowed to react withwater to produce hydrogen iodide and magnesium oxide and the hydrogeniodide thus formed is decomposed into hydrogen and iodine. Theby-produced magnesium oxide, ammonia, water and iodine which remainafter recovery of hydrogen are recycled to the relevant reactionsystems.

These elementary reactions proceed smoothly at temperatures below 800°C, the reaction products which occur therein are easily separated andrecovered and the by-products which simultaneously occur therein can berecycled to relevant elementary reaction systems, enabling hydrogen tobe produced easily from water on a commercial scale.

Other objects and other characteristic features of the present inventionwill become apparent from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has evolved from a detailed investigation for anovel reaction cycle for the thermochemical production of hydrogen. Inthe course of this investigation, the basic reactions involving thereactions of halogen containing compounds have been studied and it hasbeen found consequently that, of the halogens, chlorine exhibitsextremely high reactivity while the reactivity of bromine and that ofiodine decrease with the increasing atomic numbers thereof. Theinvestigation of the thermal decomposition behavior of hydro-halogenacids has led to the assertion that hydrogen iodide is easily decomposedinto hydrogen and iodine while the thermal decomposition property ofhydrogen bromide and that of hydrogen chloride are lower in the ordermentioned.

The conclusion, therefore, is that favorable results cannot be expectedfrom an attempt to obtain hydrogen from hydrogen chloride but that it isadvantageous to use either hydrogen iodide or ammonium iodide becausehydrogen is easily recovered therefrom by thermal decomposition or othermeans.

From the foregoing findings, it has become apparent that a reactioncycle including 2HI → H₂ + I₂ as the basic reaction and a reaction cycleincluding 2NH₄ I → 2NH₃ + 2HI → 2NH₃ + H₂ + I₂ as the basic reaction areadvantageous for the hydrochemical production of hydrogen. The studybehind the present invention concentrated on the development of reactioncycles including these basic reactions and it consequently has beendiscovered that the reaction for generating hydrogen through the thermaldecomposition of ammonium iodide can be carried out quite effectively byusing barium iodide, ammonia and carbon dioxide in combination and thatthe barium iodide, carbon dioxide, ammonia and iodine which areby-products in the reactions can be recycled to the relevant elementaryreaction systems.

It has also been found that when magnesium oxide or tri-iron tetraoxideis allowed to take part in these basic reaction cycles, hydrogen can beobtained through the thermal decomposition of hydrogen iodide. Thepresent invention has been accomplished consequently.

The reaction cycles which underlie the method of the present inventionare as illustrated in the following flow diagram. ##STR1##

First, a description will be made of the basic reactions which underliethe method of this invention.

In the first step, Reaction Formula (1), barium iodide is dissolved in asaturated ammonium iodide solution (63.9 wt%) at 25° C and, while theresultant solution is kept at about 80° C under simultaneous agitation,ammonium carbonate is added thereto, with the result that there occursprecipitation of barium carbonate. The precipitated barium carbonate caneasily be separated from the mother liquor by filtrating the reactionmixture while hot. When the filtrate remaining after the separation iscooled, the ammonium iodide present therein crystallizes out. Thus,ammonium iodide of a substantially equivalent weight to barium carbonateis obtained in the form of a precipitate.

In the reaction of Formula (1), separation and recovery of bariumcarbonate and ammonium iodide are effected easily because the reactionproceeds in the system in which ammonium iodide remains in a saturatedstate. To be more specific, ammonium iodide of an equivalent weight isobtained in the form of a precipitate by merely adding barium iodide toan ammonium iodide solution which is saturated when cold, allowing thereaction to proceed at an elevated temperature, separating the resultantbarium carbonate by filtration and cooling the filtrate. Therefore, thereaction obviates the necessity of including an operation forconcentration through vaporization of water despite the fact that thisreaction is an aqueous solution reaction. From the standpoint ofprocess, therefore, this reaction proves highly advantageous in terms ofheat balance (in consideration of the fact that the concentration ofaqueous solution by evaporation entails a heavy loss of latent heat ofwater through evaporation and can be a major cause for degraded heatefficiency).

In the second step, Reaction Formula (2), the barium carbonate obtainedin the first step is allowed to react with iodine at 750° - 800° C. Thecarbon dioxide gas generated in this reaction is absorbed in the ammoniawater and consequently is separated from oxygen and, at the same time,there is obtained an aqueous ammonium carbonate solution to be suppliedto the first step. In this case, since barium carbonate is in a solidstate and the formed barium iodide is in a liquid state at thetemperature at which the reaction proceeds, the barium carbonate and thebarium iodide are readily separated and the reaction proceeds smoothlyinsofar as the reaction tube is held in an inclined position. It shouldbe noted that although the corroding property on quartz of barium iodideincreases with the increasing temperature, practically no problem ofcorrosion arises insofar as the reaction temperature falls in theaforementioned range.

Recovery of hydrogen from the ammonium iodide formed in the first stepis accomplished by subjecting said ammonium iodide to direct heating orby allowing it to react with tri-iron tetraoxide or magnesium oxide.First, the recovery of hydrogen by said direct heating of the ammoniumiodide will be explained.

In the third step, Reaction Formula (3), the ammonium iodide obtained inthe first step is heated to 500° - 700° C so as to undergo thermaldecomposition. When the gas produced by the decomposition is cooled,iodine is crystallized out. The iodine thus precipitated is separatedand supplied to the second step. The gas remaining after the separationof iodine is introduced into water to have the ammonia componentabsorbed in the water and the hydrogen separated therefrom. The ammoniawater thus produced and the carbon dioxide gas produced in the secondstep are allowed to react with each other and produce ammonium carbonatesolution, which is supplied to the first step.

The separation of iodine from the gas which is generated in the thirdstep may be accomplished by subjecting said gas in its original form toa treatment by a method which utilizes a special permselective membranecomposed preponderantly of alumina or a method which utilizes thecharacteristic factor that iodine has a much greater mass than othercomponents of the gas.

Next the recovery of hydrogen from the ammonium iodide through thereaction of said ammonium iodide with tri-iron tetraoxide will bedescribed.

In the fourth step, Reaction Formula (4), the ammonium iodide obtainedin the first step and tri-iron tetraoxide are intimately mixed and theresultant mixture is gradually heated to 500° C, with the result thatthere is produced ferrous iodide.

Commercially, a vertical reaction furnace is used, wherein tri-irontetraoxide is introduced downwardly from the head thereof and hydrogeniodide is sublimated upwardly from the bottom thereof and the twocompounds are consequently brought into continuous counterflow contacttherein, with the result that the reaction of said two compounds willproceed efficiently. The ammonia and water which are produced in saidreaction are used for the recovery of carbon dioxide gas in the secondstep.

In the fifth step, Reaction Formula (5), the ferrous iodide obtained inthe fourth step is caused to react with steam at 500° - 600° C. Thisreaction, similarly to the reaction of the fourth step, can be effectedin the manner involving the counter-flow contact of the reactants. Wherethe resultant hydrogen iodide contains steam, said ferrous iodide can beutilized as the desiccant for said wet hydrogen iodide. The ferrousiodide which has consequently absorbed water can be used in itsunaltered form in the present step of hydrolysis.

In the sixth step, Reaction Formula (6), the hydrogen iodide obtained inthe fifth step is heated to 500° - 800° C so as to be decomposed intoiodine and hydrogen. The iodine and hydrogen are separated from eachother by their difference in weight. Or the hydrogen is separated fromthe iodine by means of a special separating diaphragm utilizing the factthat the radius of the iodine atom is notably large. The iodine thusrecovered is returned to the second step. Otherwise, this separation maybe effected by a method utilizing the fact that hydrogen iodide isdecomposed into hydrogen and iodine when it is exposed to a light of2540 A at room temperature.

Now the method for recovering hydrogen from the ammonium iodide obtainedin the first step by the reaction of said ammonium iodide with magnesiumoxide will be explained.

In the seventh step, Reaction Formula (7), the ammonium iodide obtainedin the first step and magnesium oxide are intimately mixed and theresultant mixture is gradually heated to 500° C, with the result thatthe reaction will give rise to magnesium iodide. The ammonia and waterwhich are simultaneously generated in the reaction are used for therecovery of carbon dioxide gas in the second step.

In the eighth step, Reaction Formula (8), the magnesium iodide obtainedin the seventh step is allowed to react with steam at 400° - 500° C,with the result that the hydrolysis of the magnesium iodide will proceedquickly. When the formed hydrogen iodide contains steam, magnesiumiodide can be used as the desiccant for the wet hydrogen iodide. Themagnesium iodide which has consequently absorbed water can be used inits unaltered form in the present step of hydrolysis.

The hydrogen iodide gas evolved in the eighth step can be decomposed byheating at temperatures of 500° - 800° C or by irradiation of light andseparated into hydrogen and iodine as by using a separating diaphragm inmuch the same way as in the sixth step. The iodine consequently obtainedis returned to the second step.

For the purpose of commercial production of hydrogen by thethermochemical decomposition of water, it is desirable to use a reactioncycle such that the elementary reactions thereof proceed smoothly andthe overall amount of hydrogen formed thereby is large. What is moreimportant is that there should be established a reaction cycle such thatthe consumption of energy is small, elements or compounds free fromtoxicity and corrosiveness are used as media, the reaction products areeasy of separation and recovery, and the by-products can be recycled tothe relevant elementary reaction systems.

When the method of the present invention is considered from the point ofview mentioned above, it is noted that since the reaction in the secondstep proceeds at 750° - 800° C, that in the third step proceeds at500° - 700° C, that in the fifth step proceeds at 500° - 600° C, thethermal decomposition of hydrogen iodide proceeds at 500° - 800° C andthe reactions in the other steps proceed invariably below 500° C, theenergy consumption for all the elementary reactions involved thereinwill suffice with the use of solar heat or heat from an atomic pile.Since iodine, ammonia, barium, magnesium, iron, etc. which are availableabundantly as natural resources and do not exhibit any appreciabletoxicity and corrosiveness are used as the media, there is absolutely nopossibility that the reaction tanks will be corroded or the hydrogenrecovered will contain noxious impurities. The reaction products whichoccur in the individual elementary reactions can easily be recovered andseparated and the by-products which simultaneously occur in saidreactions can be recycled to the relevant elementary reaction systems.Thus, the evolution of hydrogen can be effected continuously by themethod of this invention by having the reaction systems replenished fromtime to time with water, which is about the only substance consumed inthe reactions.

It should be noted particularly that the reactions in the individualsteps are not always required to proceed to 100% and that they are notimpeded by the presence of undecomposed or unreacted compounds such as,for example, of ammonium iodide, hydrogen iodide, tri-iron tetraoxideand magnesium oxide in the respective steps and that these undecomposedor unreacted compounds sooner or later participate in the relevantreactions in the course of circulating through the reaction systems,implying that the method of this invention practically produces one moleof hydrogen from one mole of water.

According to the present invention, the elementary reactions involvedtherein proceed without any harsh conditions, the by-products arerecycled to the relevant elementary reaction systems, the heat energyconsumption is small and hydrogen can be commercially produced with easefrom water as described above.

The present invention will be described specifically herein below withreference to preferred embodiments. It should be understood that thepresent invention is not limited to these examples.

EXAMPLE 1

First step -- In 1 kg of 63.9 wt% solution of ammonium iodide, 200g ofbarium iodide was dissolved. The resultant solution was heated to 80° Cand, while under agitation, a saturated solution of ammonium carbonatewas added thereto until barium carbonate was completely precipitated.The precipitated barium carbonate was separated by filtering thesolution while still hot. Thus 100g of barium carbonate was obtained.When the filtrate was cooled to 25° C, there was obtained an almostequivalent weight (about 150g) of ammonium iodide.

Second step -- An alumina reaction tube was packed with 100g of thebarium carbonate obtained in the first step and then set in a positioninclined so that the inlet side of the tube fell in a slightly lowerlevel. Through the inlet side of said reaction tube, iodine gas was fedat a rate of about 12 g/min. to have the reaction proceed at about 750°C for 30 minutes.

The formed barium iodide melted itself and flowed out toward the inletside of the reaction tube. Thus was formed 140g of barium iodide.Through the outlet side of the reaction tube, the generated gas was ledinto aqueous ammonia, so that the carbon dioxide component thereof wasabsorbed therein to give rise to an aqueous ammonium carbonate solutionand at the same time the oxygen component thereof was separated.

Third step -- A quartz tube was packed with 150g of the ammonium iodideobtained in the first step and set in position in a vertical electricfurnace. The raw material unit thereof was heated to 500° C so that thesolid ammonium iodide was gradually sublimated. The resultant vapor wasled through a zone heated to about 700° C so as to undergo thermaldecomposition. The gas resulting from the decomposition was sent througha condenser to separate iodine and then passed through water so that theammonia gas was absorbed by water and consequently converted intoaqueous ammonia and at the same time hydrogen gas was separated. Thus,33g of iodine was obtained and 2.9 liters of hydrogen was recovered.

EXAMPLE 2

First step -- By following the procedure of Example 1, barium carbonateand ammonium iodide were obtained by causing barium iodide to react witha saturated solution of ammonium carbonate.

Second step -- Barium iodide and an aqueous ammonium carbonate solutionwere obtained by allowing the barium carbonate obtained in the firststep to react with iodine in the same manner as in Example 1.

Fourth step -- A thorough mixture of 30g of tri-iron tetraoxide and 150gof ammonium iodide was set in a quartz reaction tube and heated to 500°C at a rate of temperature increase of 20° C/min. and held at thattemperature for 30 minutes. The gas generated consequently was cooled toroom temperature to separate iodine in the form of crystals. Thus wereobtained 90g of ferrous iodide and 25g of iodine.

Fifth step -- The ferrous iodide, 90g, obtained in the fourth step washeated to 600° C and allowed to react with steam (fed at a rate of about350 mg/min.) for 40 minutes. The gas consequently generated wasdehydrated with ferrous iodide to produce a substantially dry mixed gasconsisting of hydrogen and hydrogen iodide. Thus were obtained 14.4liters of the mixed gas (made up of 2.0 liters of hydrogen and 12.4liters of hydrogen iodide) and 21g of tri-iron tetraoxide.

Sixth step -- The hydrogen iodide, 12.4 liters, obtained in the fifthstep was thermally decomposed at about 650° C and separated into iodineand hydrogen by use of a permselective membrane composed preponderantlyof alumina. Thus were obtained 1.6 liters of hydrogen and 18g of iodine.

EXAMPLE 3

First step -- Barium carbonate and ammonium iodide were obtained byallowing barium iodide to react with a saturated solution of ammoniumcarbonate by following the procedure of Example 1.

Second step -- The barium carbonate obtained in the first step wasallowed to react with iodine in the same manner as in Example 1 toproduce barium iodide and an aqueous ammonium carbonate solution.

Seventh step -- A thorough mixture of 20g of magnesium oxide with 150gof the ammonium iodide obtained in the first step was set in a quartzreaction tube, heated to 500° C at a rate of temperature increase of 20°C/min. and held at that temperature for 30 minutes. The ammonia andwater consequently formed were used in conjunction with the carbondioxide gas formed in the second step to produce ammonium carbonate.Besides, 56g of magnesium iodide was simultaneously formed in thereaction tube.

Eighth step -- The magnesium iodide, 56g, obtained in the seventh stepwas heated to 500° C and allowed to react with steam (fed at a rate ofabout 350 mg/min.) for 30 minutes. The gas consequently generated wasdehydrated by using magnesium iodide, with the result that 9.0 liters ofsubstantially dry hydrogen iodide and 8g of magnesium oxide wereobtained.

Sixth step -- The hydrogen iodide, 9.0 liters, obtained in the eighthstep was thermally decomposed at about 650° C and then separated intoiodine and hydrogen by means of a permselective membrane made ofalumina. Thus were obtained 1.1 liters of hydrogen and 13g of iodine.

What is claimed is:
 1. A method for the production of hydrogen by thethermochemical decomposition of water, which method comprises:a.preparing a mixture of barium carbonate and ammonium iodide by reactingbarium iodide, carbon dioxide, ammonia and water; b. reacting the bariumcarbonate obtained in step (a) with iodine, thereby producing bariumiodide, carbon dioxide and oxygen; c. thermally decomposing the ammoniumiodide obtained in step (a), thereby producing hydrogen, iodine andammonia; d. recycling the barium iodide and carbon dioxide obtained instep (b) to step (a); e. recycling the ammonia obtained in step (c) tostep (a); and f. recycling the iodine obtained in step (c) to step (b).2. The method according to claim 1, wherein step (a) is conducted in asaturated solution of ammonium iodide.
 3. The method according to claim1, wherein the reaction of step (b) is conducted at 750° to 800° C. 4.The method according to claim 1, wherein the reaction of step (c) isconducted at 500° C to 700° C.